A work machine, such as a construction work machine, an agricultural work machine or a forestry work machine, typically includes a prime mover in the form of an internal combustion (IC) engine. The IC engine may either be in the form of a compression ignition engine (i.e., diesel engine) or a spark ignition engine (i.e., gasoline engine). For most heavy work machines, the prime mover is in the form of a diesel engine having better lugging, pull-down and torque characteristics for associated work operations.
An IC engine in a work machine provides input power to a transmission, which in turn is coupled with and drives the rear axles through a rear end differential. The transmission, rear end differential and rear axles are sometimes referred to as the “rear end” of the work machine. The transmission typically is attached to the front of and provides input power to the rear end differential. The rear end differential provides ground power to the two rear axles. In the case of an agricultural work machine, the rear end differential also usually includes at least one power take-off (PTO) shaft extending rearwardly within the three point hitch arrangement at the rear of the tractor.
With a work machine as described above, it is common to provide a transmission with multiple shift ranges. Typically one shift lever is used to shift between multiple gear ranges (e.g., A, B, C and D gear ranges), and a second shift lever is used to shift between discrete gear pairs within each range (e.g., 1, 2, 3 or 4). The assignee of the present invention also markets a “PowerShift” series transmission in which at least one shift lever need not be foot clutched to shift “on-the-fly” during use. Variants of the PowerShift transmission go back to the 4020 series tractors manufactured in the 1960's.
Another type of transmission used in a work machine is an infinitely variable transmission (IVT) which provides continuous variable output speed from 0 to maximum in a stepless fashion. An IVT typically includes hydrostatic and mechanical gearing components. The hydrostatic components convert rotating shaft power to hydraulic flow and vice versa. The power flow through an IVT can be through the hydrostatic components only, through the mechanical components only, or through a combination of both depending on the design and output speed.
It is desirable to provide a transmission with a park feature, which can be configured as a “park lock” or a “park brake”. A park lock is a mechanical device, and a park brake is a friction device. A park lock utilizes a compliant member in the actuation linkage since the mechanical locking teeth can “butt”. If the locking teeth butt, the compliant member allows the operator to fully engage the actuation linkage in the locked position while the teeth are butted. The teeth will engage when a small motion of the tractor allows rotation between the two halves of the park lock. The compliant member in the actuation linkage then moves to the locked position.
A conventional park lock is in the form of a park pawl. The pawl is basically an arm with teeth at one end. The pawl operates perpendicular to the transmission shaft centerlines. Space for the pawl and it's actuation linkage is required outside the transmission shafts, which can be a limitation or not even possible in some applications.
What is needed in the art is a transmission with a park feature which is effective, smaller, reliable and easy to operate.